Ground clearance for Tube Framed street/track car

I'm to the point on my MG build that I am starting out on the front and rear subframes and need to decide on my designed in chassis ground clerance. Currently the design is for 4" at ride height, with 2" of designed bump until the bump stops. The car will be driven at autox, track days, and occasionally on nice roads. The bottom of the car will be completely flat, with all electrical, fuel, and exhaust above this point. This will make the lowest part of the car the chassis, and I plan to put oak blocks on the corners and down the middle to allow the chassis to rub. The question is is 4" enough, or to much? I was wondering what people end up with on their tube frames, or on locosts.

Thanks, Daniel

Several of the Locosts run at 3.5" and one even lower so I guess you can run 4". I am told that anything under 5" can be a problem with the traffic bumps.

Are you including the rub blocks in your clearance figures?

It depends on what tires and how much bump you will be running and your suspension geometry.

You told us 2" of bump, and a little more as it compresses the stop that doesn't count. I assume that you will have enough spring to stay off the bump stop in normal cornering and will only be contacting it when you hit irregularities.

After you know what tire, and how much bump travel, look at your suspension settings. There may be a range where it works well, a range where it works less well, and an outer range where it's awful. If you are modifying pickup points, adapt them to your planned ride hight.

If the car is truly a sunday-driver (competition and very occasional "nice roads") I'd not consider speed bumps and driveways in my calculations unless some of those "nice roads" aren't so nice with frost heaves or potholes.

Oh, and I run a Locost with less than 2" clearance under my oil pan skid plate, and less than 2.5" at the frame edge. But I also run 20x9x13 tires. You're going to have a hard time finding street tires, or Dot-R tires that short.

I think I'm running just under 4" for street on my MG Midget. Avoid speed bumps and steep driveways! I always thought it'd be cool to run a taller tire/wheel combination for the street with a smaller set for the track/autocross. Problem is, you have to get the tall to start off with for the track/autox purely due to tire availability unless you want the NT01's or Azenis in 205 or 195 width.

Brust wrote: I think I'm running just under 4" for street on my MG Midget. Avoid speed bumps and steep driveways! I always thought it'd be cool to run a taller tire/wheel combination for the street with a smaller set for the track/autocross. Problem is, you have to get the tall to start off with for the track/autox purely due to tire availability unless you want the NT01's or Azenis in 205 or 195 width.

That's what I do on the Sonett, run the taller 15" rims and tires on the street and custom 13's for Auto X

About 3.5 " clearance, I point one exhaust clamp down as a " feeler".

Some paddocks ( KilKare , for one ) are pretty rough!

Thanks for all the tips. I'm going to be running 16" rims with 225/45/16's on them, and don't plan on having a dedicated set of race rims. I will probably just run the same tires street and track (probably azenis or some other ultra high performance summer only tire). I will probably stay at 4", although I might bump it down to 3.0 or 3.5.

Thanks,

Daniel

Another thing to keep in mind is that you should account for having ground clearance with NO tire and full suspension compression (assume bump stops gone/completely compressed). That way if you lose a tire on the track you would still roll on the rim instead of dragging the chassis (assuming you're still on relatively flat pavement). So, if you've got a tire that's got a 3" sidewall, you'd want ~3.5" of ground clearance at full suspension compression. Some guys will say this is hogwash, "What're the odds you're going to lose a tire AND compress the suspension?"....but I say if you're building from scratch this is good design practice.

Bryce